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In regards to frankenmotors, i just put one together. IF you can find a 2.5 as easily as a 2.2 to use the bottom end on, consider it an option.

The one i put together with dude had sat in the back of a car and had rust on the cylinders Shall i say that it runs reliably and does not smoke! this was in a 95 using the stock intake and heads and ecu.

witht this engien, there is the option for delta cams and headers as it already has dual ports and rocker valvetrain.

So what i am saying is that a frankenmotor si seasy to do, and don't worry about cylinders or the bottom end. Do be certain if any donor ej25 had been overheated or not.

What difference does the frankenmotor give with the EJ25D bottom end vs. the EJ251 bottom end? (with 91-95 2.2 heads, porting, cams, & headers)
I.E. - which one gives a higher compression anyone have approximate CR numbers?, & 91 octane would be required with said block?

run any 2.5 piston you whant i like the sti pistons for the destroker as they are resably tough and easy to find for cheap butt it works good with 2.5d pistons or 2.51 pistons but they are weaker. I can biuld a destroker engine with used parts for under 150$ so longevity is erelavent but of the 4 i built all are still runing strong. The prototype destroker has 6 years of buggy and offroad use on it

If you are planning a turbo build I would not start with the 96 legacy harness then. Start with a newer harness and computer that are user tunable without the use of a piggy back.

A harness out of an 02 WRX would make your life much easier down the line as you upgrade or change things around. Look up open source tuning for Subaru's. The extra money you spend getting the harness and ECU will be more than saved by not having to buy piggy back or stand alone aftermarket ECU's to work with the 96 legacy harness.

As far as the 2.5l short block, use a phase 2 block and crank because the rod journals are the right size for the later 2.5l turbo rods and aftermarket ones. The EJ25d has smaller rod journals and the thrust bearing on 3 instead of 5 like the Phase 2's.

Figure out the EJ22e's head chamber cc and pick a piston with a deep enough dish to compensate. You may have to go aftermarket, but a cheap option would be stock STi takeouts from a brand new block that someones putting forged pistons in.

This is assuming you're using a pre-EA82 body, because that's the only reason to run the ej22e heads. Otherwise go DOHC and do a proper turbo engine build up.

I am learning more and more about all these frankenmotors, what i was thinking was a destroker was not quite the parts I thought it was, but ya..
Seems the old '96 2.2 heads would only be good for a cheaper N/A 2.5 frankenmotor build..